Functional capillary density is variable in both normal and diseased tissue. When this parameter is defined as the number of capillaries that possess red blood cell transit, changes in functional capillary density reflect mechanisms that modulate the entrance of red blood cells into the capillaries. These mechanisms have anatomical origin, whereby the capillary diameter changes, and may also be hydrodynamic, when flow conditions prevent red blood cells from entering a capillary branch. An intrinsic feature of both processes is that capillaries undergo lumenal changes. Capillary lumen is determined by a composite of mechanical and cellular factors, where intravascular pressure is one of the principal determinants affecting diameter as a consequence of the elastic properties of the capillary/tissue system. The hydration of the surrounding tissue and the cellular volume regulation of the endothelium are additional factors. There is increasing evidence that capillaries possess contractility and that this phenomenon has spontaneous components. Consequently, functional capillary density is the resultant of both passive and active processes present at the level of individual vessels.